The senior businessman appointed by Prime Minister Tony Abbott to review Australia's renewable energy target has been the subject of a secret internal investigation into his role as a former director of a firm involved in Australia's worst foreign bribery scandal.

The investigation findings were sent earlier this month to the Reserve Bank board and deal with Dick Warburton and his fellow former Note Printing Australia directors' knowledge and handling of Note Printing Australia's sanctions-busting trip to Iraq in 1998.

On Monday, Mr Abbott personally approved the appointment of Mr Warburton to review the nation's renewable energy target, despite serious questions about the role of Mr Warburton and his fellow former NPA directors in overseeing a company that police allege engaged in repeated foreign bribery.

Questions: Dick Warburton. Photo: AFR

The federal police have alleged that during the period Mr Warburton was an NPA director, the company repeatedly engaged in foreign bribery across Asia.

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The recent inquiry into the former NPA board - which Mr Warburton was a part of between 1998 and 2007 - was conducted by forensic investigators from KPMG. They investigated Mr Warburton and his two fellow NPA directors' handling of NPA's attempt to strike a banknote deal with Saddam Hussein in 1998 in potential breach of UN sanctions.

On Wednesday the Reserve Bank refused to release the confidential KPMG inquiry or answer questions about it. Mr Warburton also declined to say whether he had been interviewed for the KPMG probe, saying that was a matter for the Reserve Bank.

He said he had spoken directly to Mr Abbott about his appointment to the renewable energy review and the Prime Minister had ''obviously not'' been concerned about the allegations.

''As far as I know they are well aware of it [the KPMG probe], they do strong due diligence, I'm sure it would have been taken into account,'' he said.

''They clearly have the same feeling I have, that there was not much credibility in the report'' - a reference to the report last September by Fairfax Media and the ABC that revealed NPA's attempts to strike a secret deal with the regime of Saddam Hussein.

The KPMG report was ordered after Fairfax Media revealed that an NPA file faxed to one of Mr Warburton's fellow directors showed that NPA had arranged for a ''front'' man in Iraq to disguise the firm's dealings with Saddam Hussein.

Opposition Leader Bill Shorten questioned the appointment of Mr Warburton to the key role, given his ties to the foreign bribery scandal.

''The onus is on Tony Abbott to now explain whether Mr Warburton's appointment is appropriate,'' he said.

Greens leader Christine Milne called for Mr Warburton to be stood aside from the renewable energy review.

''These serious allegations, if proven, go straight to the integrity and competence of Dick Warburton as a director of Note Printing Australia. On top of his climate scepticism, these allegations if proven cast even more doubt over his suitability to head the RET Review given that it will have multibillion-dollar implications for Australian business and investment,'' she said.

''Tony Abbott should drop him from the RET Review now. He should also explain whether he or any of his ministers knew of this investigation when Mr Warburton was appointed to the RET Review.''

A spokeswoman for the Prime Minister said: ''Mr Warburton had complied with all due diligence processes - processes which have been applied by governments of all persuasions''.

Mr Warburton's appointment to lead the review of the RET, announced earlier this week, has drawn severe criticism from environmental groups who have questioned whether the self-described climate change sceptic is an appropriate choice.